Field of the Invention
The present invention relates to a robot control method and a robot control apparatus capable of learning a commanded trajectory based on a trajectory error between a commanded trajectory and an actual trajectory and controlling the commanded trajectory.
Description of the Related Art
In recent years, intensive efforts have been made to realize a robotic system including a robot capable of quickly performing a complicated assembling process as well as human hands are capable. Ideally, when a target trajectory is specified by a commanded trajectory, a robot is driven exactly along the target trajectory. However, in practice, a trajectory tracking error may occur between the actual trajectory and the commanded trajectory. Besides, as the speed of driving the robot increases, the trajectory error increases.
To reduce such a trajectory error, it has been proposed to perform a learning control such that a next operation result is learned based on a previous operation result (Japanese Patent Laid-Open No. 2006-110702). In the technique disclosed in Japanese Patent Laid-Open No. 2006-110702, a robot is driven according to a commanded trajectory, and an actual trajectory is measured using a sensor or the like. Using a proportional-integral-derivative (PID) compensator, a correction of the trajectory is calculated based on the trajectory error between the actual trajectory and the commanded trajectory, and a next commanded trajectory is generated by adding the correction of the trajectory to the current commanded trajectory. By learning the commanded trajectory each time the robot is driven in the above-described manner, it is tried to achieve the actual trajectory of the robot as close to the target trajectory as possible. An assembling process performed by a robot is usually a repetition of basic operations, and thus it is rather easy to achieve an expected improvement by the learning in the above-described manner.
However, even in the technique disclosed in Japanese Patent Laid-Open No. 2001-182785, there is a restriction on the reduction in trajectory error. That is, the trajectory error may be caused by an operation delay in a servo control system and a vibration of a robot. Joints of a robot are driven by a servo control system in which a control bandwidth is not infinitely wide but is limited, and the limitation on the control bandwidth makes it difficult for the actual trajectory to exactly follow the commanded trajectory, and thus a trajectory error may occur. Furthermore, the rigidity of joints and the rigidity of a main body of the robot are not infinitely high but are limited, and thus any movement of the robot may cause a vibration with a natural frequency to occur, which may result in a trajectory error. In particular, a trajectory error caused by a vibration is characterized in that the vibration has a large amplitude in a frequency range around the natural frequency, and an output has a phase different by 180° from that of an input.
To ensure high stability in the servo control system, the control frequency band is usually set to be lower than the natural frequency of the robot. However, this causes the servo control system to have a delay in response in a low frequency range and to cause the robot to have a vibration in a high frequency range, which may be main causes of the error. A similar situation may occur also in a control loop in a learning control system. Thus, to ensure high stability in the learning control, the control frequency band is set to be lower than the natural frequency of the robot. However, as a result, the robot may have a vibration in a range higher than the control frequency band of the learning control system which makes it difficult to reduce the trajectory error caused by the vibration of the robot. Therefore, it is difficult to reduce the trajectory error by the learning control using the technique disclosed in Japanese Patent Laid-Open No. 2001-182785.
Another possible control method is to model a robot and configure a compensator used in the learning control so as to operate according to an inverse model thereof. However, practically, an inevitable error occurs in modeling, which may create an adverse influence. This is known as a spill-over problem. For the reasons described above, there is a restriction on designing a compensator to be capable of reducing the vibration of the robot, and there is a trajectory error that is difficult to reduce by the learning control.
To handle the above-described situation in which it is difficult to reduce the trajectory error, the gain of the compensator may be reduced to achieve high stability in the learning control. However, the reduction in gain results in a reduction in speed of adjusting the trajectory error. As a result, a large number of iterations of learning is necessary, which results in a reduction in the learning speed. Besides, in the learning control, noise may occur in the middle of measurement of the actual trajectory, and such noise may cause the trajectory error to become worse compared with a previous trajectory error. In this case, in the learning control according to the technique disclosed in Japanese Patent Laid-Open No. 2001-182785, the learning control is continued from a degraded state, and thus a large number of iterations of learning is necessary, which results in a reduction in the learning speed.